Anticorrelated: Sheila's attraction to Hubby implies Hubby has qualities which make him appealing to women, which makes Wifey less likely to cheat.

Correlated because cheating by one partner implies less mate-guarding by the other. A partner who doesn't bother to mate guard is less concerned about losing that partner, and thus more likely to cheat.

Also both partners are more likely to cheat if the passion has died down in the relationship, or if they're in the midst of a conflict.